Yeah it's a physical limitation.
I've got the faster i7 cpu, and even playing 2.7K GoPro footage it ramps up to ~80c.
4K just kills it.
Part of the thermal design presses the heat sinks onto soft conductive pads which transfers the heat to the aluminium bottom. Can make working on your lap uncomfortable, but also means one of those cooling pads should be quite effective.
It's not a great design for heavy users, but it was a beautiful design, and unlike everything else, it doesn't vacuum up air from underneath and clog up with dust in 6 months.